The present application relates to electromagnetic structures that can function as antennas for transmitting or receiving electromagnetic energy and as waveguide probes in cavities for injection or extraction of electromagnetic energy.
It is well known in the electromagnetic arts that efficient, linear antennas are usually constructed from elements having lengths that are significant portions of a free-space wavelength at the operating frequency. It is also known that if those lengths are made equal to integer multiples of one quarter wavelength, standing waves may be induced in the antenna. It is also understood that operation of an antenna at one of its self-resonance frequencies, if possible, is desirable to increase antenna efficiency. At the self-resonant frequencies, standing waves are produced on antennas and the reactive component of the feedpoint impedance is zero. This efficient operation contrasts with the familiar "matched" operation where the impedance of an antenna is conjugately matched by an external network to the impedance of a transmitter or receiver to improve performance. Reactive power losses are experienced both in the antenna and in the matching network, when a matching network is used, so that overall system efficiency is not maximized. It is also established that horizontally polarized electromagnetic waves suffer greater ground wave propagation losses than do vertically polarized waves. Therefore, vertically polarized waves are preferred over horizontally polarized waves for communication over the surface of the earth.
It is recognized that a vertical antenna having a length equal to one quarter of a wavelength at the operating frequency provides a desirable vertically polarized, omnidirectional radiation pattern. However, because wavelength increases inversely with operating frequency, the length, i.e., the height, of such an antenna becomes unmanageably long at frequencies below about 1 MHz. As a consequence of the long wavelengths below 1 MHz, various antenna structures have been employed at those frequencies. Generally, those antenna structures are physically large, may not necessarily produce the desired vertically polarized signal, and are not self-resonant. Therefore they are inherently inefficient as well as being unwieldy.
The goal of constructing a physically small, but self-resonant (and therefore efficient) antenna or waveguide probe has eluded electromagnetic arts specialists for over three-quarters of a century. An antenna or other electromagnetic structure is electrically small when its physical size is small relative to the free-space wavelength at which it operates. Thus, at the lower end of the radio frequency spectrum where wavelengths are relatively long, a physically large electromagnetic structure may still be electrically small. As used here, the term "electrically small" means that the physical dimensions of an electromagnetic structure, measured in terms of free-space wavelengths, at the operating frequency, are small, whether or not the structure may be electromagnetically self-resonant.